1. Field of the Invention
The invention relates to an explosive safe-arming system for use in a perforating gun for oil and gas well wireline operations.
2. Description of the Prior Art
The perforating guns most commonly used in present-day wireline service operations are typically comprised of an elongated fluid-tight body or so-called "enclosed carrier" which houses one or more shaped explosive charges and the necessary accessories for selectively detonating these charges from the surface. One typical style of such carriers employs an expendable, thin-walled steel tube which has reusable upper and lower heads fluidly sealed in each end of the tubing. Other common types of enclosed carriers have heavy, explosion-resistant walls so that the carrier can be retrieved. This latter type of carrier is ordinarily provided with a removable head or an access port to accommodate the installation of the shaped charges and their associated detonating components.
Those skilled in the art will recognize, of course, that a typical shaped charge perforating gun ordinarily poses no serious hazards so long as there is either a spatial interruption in the explosive detonating train for the gun or the electrical wiring to the detonating train is suitably disconnected. Thus, the usual practice is to substantially complete the assembly of a given gun, but in some approved manner, leave the gun in a relatively safe or "disarmed" condition until just before it is to be operated. Another such safe-handling technique is simply not to install the electrically-initiated detonator for a given gun until it is being prepared for immediate operation.
It will be appreciated, however, that such typical safe-handling techniques are not entirely satisfactory. For example, when a perforating gun is disarmed by not installing the entire electrically-initiated detonator, there are still safety and logistic problems involved in handling the entire removed detonator. Moreover, where last minute electrical connections or other detailed preparations are required to ready the perforator for firing, these final steps often must be made under severe environmental conditions which can easily contribute to either a malfunction or even an unsafe or improper operation. Accordingly, it is most desirable to not have any electrical connections made in the field, but rather at the factory or in a field office.
One approach to solving the general problem of providing a safety detonator may be found in U.S. Pat. No. 3,719,144, issued on May 6, 1973, to Elvin W. Tlam. This patent discloses providing a detonator comprising a first detonator unit containing a liquid explosive mix component and adapted to fit into a conventional receptor detonating explosive, or booster, and a second detonator unit which is a syringe filled with a second liquid explosive mix component. The detonator is armed by inserting the syringe through an end opening of the first detonator unit, whereby the liquid mix component of the syringe is combined with the liquid mix component of the first detonator unit to form a complete liquid explosive. In operation, upon firing of a conventional squib, the liquid explosive formed by the two components is detonated, which in turn causes the detonation of the receptor detonating explosive, or booster. In the event that the squib would be accidentally detonated prior to the addition of the second liquid explosive mix component, the explosive force would not be sufficient to detonate the booster.
The two part detonator unit of the Tlam patent has disadvantages. The liquid explosive mix components of the Tlam detonator can only be used under low temperature conditions, on the order of 200.degree. F. or below. Additionally, the liquid explosive mix components, which are not only corrosive and toxic, are somewhat unstable, and thus hazardous to handle. Once the Tlam detonator is activated and armed by injection of the second liquid explosive mix component, the detonator may not be disarmed. This fact would cause operational disadvantages if the Tlam detonator were to be used in a perforating gun for oil and gas well applications because it is common practice to arm a plurality of perforating guns, wherein an additional number of guns are provided in the event of a misfire of one of the perforating guns. Proper firing of the desired number of perforating guns results in the retrieval from the well bore of the extra previously armed perforating guns, which could not then readily be disarmed if the Tlam detonator were used.
Furthermore, since it is often desirable to mount a plurality of perforating guns in series, wherein each of the perforating guns are in end-to-end relationship, it would be extremely impractical to gain access to the first detonator units with the various syringes, absent arming each individual detonator with the syringe, and then connecting the plurality of perforating guns in series. Additionally, a major disadvantage associated with the detonator of the Tlam patent is that if in the event well fluid present in the well bore enters the perforating gun, the presence of such fluid within the perforating gun transforms it from a device which provides for a controlled explosive force from the shaped charges disposed in the perforating gun, into an uncontrolled explosive device, a latent bomb. If such fluid were to enter a perforating gun provided with the Tlam detonator, there can be no provision for desensitization, or disarming, of the detonator due to the presence of the well fluid in the perforating gun.
Finally, the detonator of the Tlam patent cannot be used with conventional well bore perforators having an enclosed carrier with an access port disposed in the outer surface of the carrier, since the syringe unit of Tlam must be inserted through the end of the detonator rather than through the outer circumferential surface of the detonator. Thus, conventional perforating guns would have to be modified to use the Tlam detonator.
Accordingly, prior to development of the present invention there has been no safe-arming system for use with perforating guns which: is readily adaptable for use with existing perforating guns which are connected in an end-to-end relationship in series; does not require last minute electrical connections to be made in the field to ready the perforator for firing; does not necessitate the use of corrosive, toxic, and unstable liquid explosive components; may be used over a wide range of operating temperatures; may be readily, selectively armed and disarmed; does not require the installation of the entire electrically-initiated detonator in the perforating gun at the time the perforating gun is being prepared for immediate operation; is inexpensive to manufacture; and efficient and easy to use. Therefore, the art has sought an efficient, safe, and inexpensive safe-arming system for perforating guns, which can be used with conventional perforating guns, does not require the use of liquid explosive components, and can be readily, selectively armed and disarmed.